It is common practice to increase the range of aircraft by incorporating a drop fuel tank under each wing of the aircraft. The aircraft takes off with filled wing drop tanks and flies to a destination using the fuel in the drop tanks. The empty drop tanks are then jettisoned to allow the aircraft to operate at its maximum performance level. The drop tanks are jettisoned because of they generally reduce the performance of the aircraft. For example, drop tanks increase the aerodynamic drag on the aircraft and increase the load on the wings. The costs associated with replacing the jettisoned drop tanks is prohibitive. In addition, the logistics associated with storing, maintaining, and transporting the number of drop tanks required for a combat situation is extremely difficult.
To eliminate the problems associated with drop tanks, various permanent variable displacement fuel tanks have been proposed. These proposed tanks generally rely on fuel pressure to expand the tank. These proposed tanks also relied upon elastomeric bladders or a system of lanyards or webs to vary the size and shape of the fuel tank. Approaches such as these make it difficult to manage the fuel, as induced acceleration and inertial effects tend to move the fuel load in an unpredictable manner. Uncontrolled variations in the fuel load center of mass are generally unfavorable in controlling an aircraft.
In addition, the proposed variable displacement fuel tanks are not particularly suitable for use in modern high density design aircraft. The aerodynamic and space requirements for such modern aircraft has made the use of permanent variable displacement fuel tanks very difficult. For example, modern high density design aircraft require careful placement of the permanent variable displacement fuel tanks and control of the shape of the tank under all fuel load conditions to prevent fuel inertia loads from compromising the performance of the aircraft. In addition, particular attention must be given to any external portion of the tank to avoid excessive drag and aerodynamic loads.